1. Field of the Invention
The present invention relates to power amplifiers, and more particularly, to feed-forward power amplifiers.
2. Description of the Related Art
It is well known in the pertinent art that all amplifiers distort an input signal to varying degrees. The distortion typically becomes greater as the power levels are increased. When signals pass through non-linear radio-frequency (RF) amplifiers, the signals may undergo distortions resulting in a spreading of the spectrum beyond the allocated channel and a reducing of performance. The inefficiency and high cost of conventional linear RF amplifiers (e.g. Class A) have a negative effect on base station infrastructure equipment and operation. A more ideal amplifier would therefore be linear and power-efficient.
To linearize power amplifiers, feedback methods and feed-forward methods are used. However, feedback methods may cause power amplifiers to oscillate at high frequencies, and thus, a certain amount of instability can be observed in the feedback methods. Therefore, the feed-forward methods are widely used.
In a feed-forward method, an error signal is extracted and separately amplified by an error amplifier so that the error signal may be removed from an output signal of a main amplifier. Therefore, circuits are more complicated in the feed-forward methods and power efficiency is reduced due to the use of the error amplifier. In addition, a gain of the power amplifier and the distortion characteristics can vary with a supply voltage and temperature, thus limiting the amount of correction.
FIG. 1 is an exemplary block diagram illustrating a conventional feed-forward amplifier. Referring to FIG. 1, the conventional feed-forward amplifier includes a main path 111 and an error path 113.
The main path 111 includes a main amplifier 101 that amplifies a power level of an input signal Vi. Due to the nonlinearity of the main power amplifier 101, an intermodulation signal may be generated at an output terminal of the main amplifier 101. An output signal of the main amplifier 101, including such intermodulation signal, is then applied to a first subtractor 109.
The error path 113 includes an attenuator 103, a second subtractor 105 and an error amplifier 107. A signal may be distorted or an intermodulation signal may be generated in the error path 113 based on the input signal Vi of the conventional feed-forward amplifier. Particularly, a third order intermodulation component IM3 may be extracted from the input signal Vi in the error path 113 of the FIG. 1.
The attenuator 103 has a gain that is a reciprocal of a gain of the main amplifier 101 in the main path 101. Therefore, when a voltage gain of the main amplifier 101 corresponds to Av, a voltage gain of the attenuator 103 corresponds to 1/Av. A carrier signal and the intermodulation signal that are amplified in the main amplifier 101 are attenuated by the attenuator 103 and applied to the second subtractor 105.
The carrier input signal Vi of the feed-forward amplifier and an output signal of the attenuator 103 are applied to the second subtractor 105. A carrier component is then removed by the second substractor 105 to generate the error signal having only an intermodulation component therein.
The error signal is applied to the error amplifier 107, and a voltage gain of the error amplifier 107 corresponds to Av. The error signal is amplified and applied to the first subtractor 109.
An output signal of the main amplifier 101 and an output signal of the error amplifier 107 are applied to the first subtractor 109. Since the intermodulation component or a distortion component is removed from the output signal of the main amplifier 101, the carrier signal may be linearly amplified.
However, the conventional feed-forward power amplifier has some disadvantages when both a signal in the main path and the error signal have high frequencies, thereby limiting the control of a gain or a phase thereof. In addition, delays of signals generated in the respective paths need to be compensated. To compensate the delays of signals, a delay path should be separately equipped. However, in such a method, the third order intermodulation components must be controlled, which is difficult.